Hydrography is the field of study which describes and maps the physical features of bodies of water, including oceans, seas, freshwater lakes, rivers and estuaries. It is a multi-disciplinary field, comprising elements of oceanography, cartography, marine geology, acoustics and other areas. The primary concern is with the mapping and documenting of features which are of significance to navigation, commercial shipping and fishing, but it also deals with oil and gas exploration, defense, marine and freshwater research and protection of the environment. A hydrographic survey of an area of ocean or freshwater can be carried out with a view to producing a map or chart detailing the points of interest, particularly navigational hazards such as submerged rocks, sandbanks and shipwrecks, but also temperature, salinity and the flow of currents. Of special importance in hydrography is the topography and nature of the seafloor.
By measuring the depth of the water at regular, known points, a hydrography map can be prepared. Determination of position in terms of latitude and longitude was, until the late 1970s, done visually, using sextants and triangulation methods, but since then electronic means and the Global Positioning System (GPS) have been used. Up to the 1930s, water depth was measured using lead weighted lines or sounding poles, after which this slow and inaccurate method was largely replaced by acoustic measuring techniques. These employ a pulse of sound sent downwards from a transmitter to echo from the floor and a receiver to capture the echo. The time taken for the echo to be received combined with knowledge of the speed of sound in water allows the depth to be calculated.
Single Beam Echo Sounders (SBES) are often used in hydrography for shallow waters and employ a single narrow pulse of sound to record the shallowest depth within the area covered, as this is of the greatest importance for navigation. Corrections need to be made to the raw data to allow for tides, the movement of the vessel and variations in the speed of sound at different depths. Multi-Beam Echo Sounders (MBES), which employ a large number of sound pulses spread out in a fan shape, achieve more comprehensive coverage and are generally used in deep water. These can produce very detailed maps, but they are more expensive than SBES and the corrections are more complicated. Side-scan sonar employs a sound source close to the sea floor that produces near-horizontal sound beams, providing high-resolution images of features that might not show clearly in SBES or MBES data.
The composition of the sea floor is of importance for anchoring, undersea construction, laying of pipelines and other marine activities. Samples can be obtained from cores, dredging or simply scooping up material to determine its type — for example sand, mud or rock. Underwater photography is also sometimes employed. This information, along with data from depth surveys and measurement of tides, can be incorporated into hydrographic and nautical maps used for navigation.
Other data not related to navigation may be recorded by a hydrographic survey for purposes of scientific research. For example, ocean surface temperatures can be measured from space by satellites that record infrared radiation received, and sub-surface temperatures recorded by thermometers. Salinity can be regarded as a measure of the amount of dissolved solid material in ocean water, and is generally measured by the electrical conductivity of the water. Ocean currents can be measured either by a current meter, a device which measures the strength of the current at a particular depth, or by using an Acoustic Doppler Current Profiler (ADCP), which can record currents across a range of depths.
Hydrographic services are provided by a number of organizations. In the USA, for example, there is the National Oceanic and Atmospheric Administration (NOAA) Office of Coast Survey, and the in the UK, the United Kingdom Hydrographic Office. The International Hydrographic Organization (IHO) has established international standards for hydrographic surveys and maps.